Pressure vessel for compressed oxygen or oxygen-containing gases



June 25,, 1940. E. ZURBRUGG 2,205,708

PRESSURE VESSEL FOR COMPRESSED OXYGEN 0R OXYGEN-CONTAINING GASES Filed Dec. 21 1957 INVENTOR 5 ATTORNEY Patented June 25, 1940 UNITED STATES PRESSURE VESSEL FOR COMPRESSED OXY- GEN OR OXYGEN-CONTAINING GASES Emil Zurbriigg, Neuhausen, Switzerland, assignor to Aluminium Industrie Aktien Gesellschaft, Neuhausen, Switzerland, a joint-stock company of Switzerland 7 Application December 21, 1937, Serial No. 180,931 In Switzerland December 23, 1936 8 Claims.

This invention is a novel pressure vessel for compressed oxygen or oxygen-containing gases. Such vessels or flasks are well known, being usually in the form of a cylinder for containing, storing and transporting the gases, having means for charging and discharging at will. While formerly such vessels or cylinders were almost exclusively composed of steel, where high pressures were involved, they have for a considerable period been composed of a light metal, thus minimizing the weight and better adapting the vessels for transportation, a notable field of utility being in the carrying of oxygen, or oxygen-containing gases, in air-craft. The light metals referred to comprise usually aluminum or its light alloys, although the same principles would apply to magnesium. The gases referred to comprise typically oxygen or oxygen-containing gases, which are capable of combining exothermically with the light metal referred to, so as to burn with the development of high temperature.

In this special field of light-weight pressure vessels for storing oxygen or oxygen-containing gases readily combinable with the wall metal, it has been found in actual practise that when such a vessel is violentlystruck or pierced, as by a projectile from a rifle or.machine gun, the wall material becomes heated at the point of impact to such a high temperature, or above its melting point, as to start ignition and combustion of the metal in the presence of the oxygen. This action is the result of the high momentum of the projectile and its energy of impact, which are converted into heat. The temperature developed initiates the combustion of the metal in the presence of the oxygen, which flows out through the puncture with high velocity, maintaining the combustion and destruction of the wall metal, and this burning being self-supporting the confiagration continues for a substantial period, ceasing only when the whole of the oxygen or gas containing it has escaped. By this time a large portion of the metal has been burned away, as much as A; or /3 of the entire weight of the container or vessel. It is an obvious fact that this burning of the. oxygen-containing cylinder or vessel, practically impossible to extinguish, is a source of great danger to nearby persons, more especially to the crew of an aircraft and to the aircraft itself, and to those engaged in rescue or first aid.

The general object of the present invention is to provide a pressure vessel of the light-weight class, for storing compressed oxygen or like gas,

which will be substantially free of the dangers and objections mentioned.

According to the present invention, to minimize the objections and dangerous actions referred to, the light-weight pressure vessel, composed of aluminum alloy or the like, is coated over the entire inside surface of the vessel wall with a relatively thin and adherent layer of a difficultly fusible and incombustible material; and it is preferably also similarly coated over the outside surface of the vessel wall. Practical demonstration has shown that a pressure vessel which has been treated as described with a suitable coating material, is unable to burn to an appreciable extent, even in the presence of pure oxygen, when pierced by a projectile, but on the contrary will behave in general the same as a steel vessel.

The pressure vessel to which this invention pertains is a stoppered gas-flask whose thin walls are of the light metal aluminum or magnesium or equivalent, and which is of ample dimensions and capacity to store a large supply of oxygen or oxygen-containing gas, chargeable into and dischargeable from it, and the use of which incurs the danger of confiagration, when once started, between the wall metal and the gaseous contents comprising the highly compressed and therefore concentrated oxygen.

In the accompanying drawing the single figure indicates an elevation view of a pressure vessel shown in upright position and broken away at two points to show better the structure of the walls thereof. In this drawing the pressure vessel, or its wall a is composed of a light metal, as an alloy of aluminum or magnesium. This wall is shown as protectively coated at its inner surface by the incombustible and difiiculty fusible coating 1). It is shown further with a similar exterior coating 0. It is to be understood that in the drawing the coatings are considerably exaggerated for purposes of illustration, and will in fact be very much thinner than the drawing indicates, in fact so thin that the coating could not be shown in its true proportions without high magnification of the drawing. A typical thickness of coating layer, of a character to be further described, is 0.02 mm. this however being only representative, as the thickness might range between 0.003 mm. and 0.5 mm., although preferably never as thick as the drawing indicates, unless by some special coating material operatively requiring relatively great thickness.

The protective layers or coatings b and 0 each consists preferably of a chemical product, such as an oxide, which can be produced directly upon the surface as by simple chemical oxidation or by anodic oxidation, which also involves chemical action, methods for either of such forms of oxidation being well understood in the chemical and electrolytic arts. The coatings hereof are not to be compared with the negligibly thin oxidation that takes place ordinarily upon the exposure of aluminum or magnesium to atmospheric air, the greatly thicker coating of the present invention being of a diiferent order, and the natural oxide skin being wholly unadapted to affordthe advantages of the present invention.

A typical example of chemical oxidation of an aluminum alloy vessel wall is the known system which may be referred to as the modified Bauer- Vogel process. According to this process the metallic object to be oxidized at its surface is im mersed for about 10 minutes in a solution, maintained at a temperature of about to 94 C., which contains 5% more or less of sodium carbonate NazCOa, with 5% more or less' of sodium chromate NazCrO4. By this process an intimately united coating of metallic oxide is left upon the treated surface, protecting it eificiently and completely from access by the contained oxygen or other gas.

If the oxidation of the metallic surface is performed by electric action, for example electrolytically or by anodic oxidation, which can be caused to produce a thicker oxide layer than by chemical oxidation, the preferred process is, besides the use of an outer cathode, to introduce into the interior of the vessel a cathode, immersed in a wet eletcrolytic bath, through which the current is passed. It is found that by operating in this manner a thicker layer of oxide may be formed in the interior of a vessel having a reduced or narrow neck, than is possible without locating a cathode also at such internal position.

Another procedure is to coat the aluminum or light metal anodically with a fluorine-containing layer, namely in a bath containing the appropriate ingredients including a suitable melt or flux.

When the oxide layer has been formed or applied as hereinabove described or otherwise it is sometimes advantageous to subject the coating layer to a subsequent treatment. One such treatment is a supplemental coating or sealing action to insure the closing of pin holes and pores. Another subsequent -treatment.,of value is to apply over the oxide layer a coating of a suitable lacquer or varnish, preferably of the type which may be fired.

I claim:

1. A thin-walled gas-tight chargeable-anddischargeable pressure vessel or flask of the kind having ample size and capacity for storing or transporting substantial supplies of a compressed oxygen-containing gas or oxygen, the same composed of a light metal, as aluminum alloy, which when subjected to high temperature in the presenceof such gas is capable of combining exothermically therewith, causing the burning of such metal; said vessel characterized in having its wall surface adherently coated with a continuous protective layer composed of a non-combustible and difiicultly-fusible coating material of relatively small but appreciable thickness which seals and prevents access of the oxygen to the wall metal; thereby largely to reduce or obviate the danger of conflagration between the Wall metal and the compressed gas if and when a burning temperature be started, as in case the vessel wall be violently struck and pierced.

2. A pressure flask as in claim 1 and wherein the protective layer covers both the interior and. exterior surfaces of the flask walls.

3. A thin-walled gas-tight chargeable-anddischargeable pressure vessel or flask of the kind having ample size and capacity for storing 01' transporting substantial supplies of a compressed oxygen-containing gas or oxygen, the same composed of a light metal, as aluminum alloy, which when subjected to high temperature in the presence of such gas is capable of combining exothermically therewith, causing the burning of such metal; said vessel characterized in having its wall surface integrally coated with a continuous protective layer composed of a non-combustible and diificultly-fusible oxide of a metal of relatively small but appreciable thickness which seals and prevents access of the oxygen to the wall metal; thereby largely to reduce or Obl iate the danger of conflagration between the wall metal and the compressed gas if and when a burning temperature be started, as in case the vessel wall be violently struck and pierced.

4. A pressure flask as in claim 3 and wherein the oxide coating is of the order of 0.01 to 0.03 mm. thick.

5. A pressure flask as in claim 3 and wherein the coating is of an oxide of the wall metal itself between 0.003 and 0.5 mm. thick.

6. A pressure flask as in claim 3 and wherein the oxide coating is the result of chemical oxidation to an appreciable thickness involving chemical action upon the surface of the wall metal.

7. A pressure flask as in claim 3 and wherein the oxide coating is the result of anodic oxidation.

8. A pressure flask as in claim 1 and wherein the protective coating is the result of anodic treatment and contains fluorine.

EMIL zURBRiiGG. 

